Improve message to user when they don't have create space permissions

[johnboyes]

If the space does not exist when the plugin is run, the plugin will create the space - but this requires the user running the plugin to have permissions to create spaces in Confluence. If the user does not have permissions to create a space, we need to give them a useful message saying:

• [x] if the space does not exist yet, the plugin needs to be run by a user with create space permissions
• [x] or alternatively get someone to create the space manually for you, and then run it again, setting the key of the space that was manually created as a Gauge configuration variable

[johnboyes]

A spanner in the works is that the free version of Confluence Cloud does not have configurable permissions

One workaround would be to tag the Gauge specs which we'll add to verify this feature as non-cloud

[johnboyes]

Fixed by #52